A superconducting cable including a heat insulation pipe housing a cable core having a superconducting conductor layer has been conventionally known. Such a superconducting cable includes, for example, a single-core cable having a heat insulation pipe housing one cable core or a three-core cable housing three cable cores in a bundle. FIG. 7 is a cross-sectional view of a three-core superconducting cable for three-phase AC transmission. FIG. 8 is a cross-sectional view of each cable core 102. This superconducting cable 100 has a construction in which three stranded cable cores 102 are housed in a heat insulation pipe 101. Heat insulation pipe 101 has a construction in which a heat insulating material (not shown) is arranged between a double pipe formed with an external pipe 101a and an internal pipe 101b, and air between the pipes 101a, 101b is evacuated. Each cable core 102 includes, from a center portion thereof, a former 200, a superconducting conductor layer 201, an electrical insulation layer 202, a superconducting shield layer 203, and a protection layer 204. Space 103 enclosed with internal pipe 101b and each cable core 102 becomes a passage of a refrigerant such as liquid nitrogen. A superconducting state of superconducting conductor layer 201 and superconducting shield layer 203 of cable core 102 is maintained by cooling with the refrigerant. A corrosion-proof layer 104 is included on an outer periphery of heat insulation pipe 101.
The superconducting cable must be continuously cooled with the refrigerant such as liquid nitrogen to maintain the superconducting state of the superconducting conductor layer and the superconducting shield layer. Therefore, a line using the superconducting cable usually includes a cooling system for a refrigerant. With this system, circulation cooling is performed in which the refrigerant ejected from the cable is cooled and the cooled refrigerant again flows into the cable.
With cooling of the refrigerant to an appropriate temperature by the cooling system, the superconducting cable can maintain the superconducting state of the superconducting conductor layer and the superconducting shield layer by sufficiently decreasing an increase in a temperature of the refrigerant due to heat generated by passage of a current or heat intrusion into the cable from the outside such as an atmosphere. When the refrigerant is liquid nitrogen, however, energy required for cooling the refrigerant to address such generated heat or heat intrusion becomes at least 10 times higher than energy handled by the refrigerant to cool the cable. Therefore, when the superconducting cable line including the cooling system for the refrigerant is considered as a whole, a coefficient of performance (COP) becomes about 0.1 or lower. Such a low COP is one of causes of a decreased application effect of a superconducting apparatus such as a superconducting cable. Thus, each of Japanese Patent Laying-Open No. 2002-130851 (Patent Document 1) and Japanese Patent Laying-Open No. 10-092627 (Patent Document 2) proposes to cool a refrigerant of a superconducting coil utilizing cold heat of a liquefied natural gas (an LNG).
On the other hand, with proceeding development of a fuel cell vehicle, there are plans to build hydrogen stations at many places in Japan for storing compressed hydrogen or liquid hydrogen to be fed to the fuel cell vehicle. The hydrogen station includes, for example, a tank for storing liquid hydrogen produced in a factory and transported or liquid hydrogen produced in the station, and a cooling system for liquefying vaporized hydrogen to maintain a liquid state. Though hydrogen can be maintained in the liquid state by cooling to an appropriate temperature with this cooling system, heat intrusion into the cable from the outside becomes large since liquid hydrogen has a cryogenic boiling point of about 20 K which is substantially different from an ordinary temperature of an atmosphere. Therefore, an enormous amount of energy is required for cooling liquid hydrogen to reduce an increase in a temperature due to heat intrusion.    Patent Document 1: Japanese Patent Laying-Open No. 2002-130851    Patent Document 2: Japanese Patent Laying-Open No. 10-092627